The lettering as I understand it represents the order in which the search tree found the move. How green the move is is the actual representation of how good the move is according to the AI. Also the best move may still lead to a decrease in win rate. Imagine if you have a 10 stone chain and a 20 stone chain both in double atari. Let’s say it’s endgame and there are no larger moves available for either player. The AI is going to suggest saving the 20 stone chain, but if it puts you behind on points, the AI will still say it’s a bad result.

Same idea here (to a degree). The approach is not a bad-move, the AI simply sees that in it’s variations, people tend to lose more by starting with that move than throwing in to the corner.